1. Field of the Invention
The present invention relates to a semiconductor laser device including a plurality of cavities and to a method of fabricating the same. Specifically, the present invention relates to a multiwavelength semiconductor laser device used as a light source typically for an optical disc device or for other devices such as an electronic device and an information processor and to a method of fabricating the same.
2. Description of the Prior Art
Currently, an optical information-storage medium is widely available as a storage medium for storing digital information for example. However, various standards are established for the optical information-storage medium, and light sources differing in wavelength are required depending on types of optical information-storage media. For example, a compact disc (CD) requires a light source having a wavelength of 780 nm in an infrared range, and a digital versatile disc (DVD) requires a light source having a red wavelength of 650 nm. Therefore, a multiwavelength semiconductor laser device for outputting a laser beam with a plurality of wavelengths is a subject of interest.
Meanwhile, in order to improve storing speed of the optical information-storage medium, increasing in power of the semiconductor laser devise is promoted. Especially, it is desired to increase the power of a semiconductor red-laser device used for a DVD device to read and write information on a DVD. One of main factors restricting the increasing in power of the semiconductor red-laser device is degradation in a cavity end face of the semiconductor laser device. The degradation is referred to as COD (Catastrophic Optical Damage) degradation and results from optical absorption caused by defects near the cavity end face. That is, when the semiconductor laser device is in a high power operation, surface recombination at an output end face and an increase in light absorption near the end face raise the temperature of the output end face, resulting in destruction of the end face.
In order to prevent the COD degradation, Japanese Laid-Open Patent Publication No. 11-186651 for example discloses to diffuse an impurity such as zinc (Zn) in an area of an active layer near a laser output end face for forming a window structure. Since the diffusion of the impurity causes intermixing in the area of the active layer near the end face, a band gap of the active layer increases near the end face. This makes it possible to keep a state substantially transparent to a laser oscillation light, even in the case where heat generation decreases the band gap of the active layer near the end face. Therefore, it is possible to suppress the COD degradation.
However, applying the conventional method of preventing the COD degradation to the multiwavelength semiconductor laser device has problems as follows. An active layer of the semiconductor red-laser device includes an aluminum gallium indium phosphorus (AlGaInP)-based material. On the other hand, an active layer of a semiconductor infrared-laser device includes an aluminum gallium arsenic (AlGaAs)-based material. The diffusion velocity of an impurity such as Zn is higher in the AlGaInP-based material than in the AlGaAs-based material. Therefore, if a diffusion condition of the impurity for forming the window structure is optimized for the semiconductor infrared-laser device, Zn diffuses into an n-type cladding layer below the active layer of the semiconductor red-laser device. Since Zn serves as a p-type impurity in a III-V group compound semiconductor, the n-type cladding layer becomes p-type, and a pn junction is eventually formed between the cladding layer and a buffer layer near an n-type substrate.
If the pn junction is formed between the cladding layer and the buffer layer, a band gap of the buffer layer which is smaller than that of the active layer may cause a turn-on in the pn junction between the n-type buffer layer and the previously intermixed window area. This lowers a forward transient build-up voltage, resulting in a cause of a leak current. Moreover, due to the turn-on, a current is no longer input in a non-intermixed active layer. As a result, light emitting efficiency decreases, a reactive current increases an operating current value, and a temperature characteristic deteriorates. Therefore, it is not possible to obtain a high power characteristic of several hundreds mW or more, and the problem arises that the reliability of the device greatly decreases.